Development of desorption ionization techniques provided perhaps the first breakthrough in the mass spectrometric analysis of fragile, non-volatile compounds such as peptides or carbohydrates. Plasma desorption, one of the first desorption ionization methods was implemented in the mid 1970's by Macfarlane, and it was successfully used for the ionization of delicate biochemical species like toxins. Plasma desorption was followed by a number of even more successful desorption ionization methods including secondary ion mass spectrometry (SIMS), liquid secondary ions mass spectrometry (LSIMS), fast ion or atom bombardment ionization (FAB) and various laser desorption techniques. Matrix-assisted laser desorption ionization (MALDI), a member of the latter group, together with electrospray ionization has revolutionized bioanalytical mass spectrometry by making the analysis of practically any kind of biochemical species feasible. MALDI is still one of the most widely used ionization methods, and certainly the most widely used desorption ionization technique.
Besides the analysis of non-volatile species, surface profiling has become an important direction of development for desorption ionization methods. Nowadays, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the most versatile tools in surface science; modern systems offer submicron resolution imaging capability. While TOF-SIMS systems were originally optimized for elemental analysis, they have since been optimized also for organic analysis. The use of MALDI for molecular imaging has recently been implemented as a soft-ionization surface analysis tool capable of providing information about the spatial distribution of peptides, proteins and other biomolecules in specifically prepared tissues.
Generally, desorption ionization (DI) has been achieved in the past by particle or photon bombardment of the sample and the mass spectra obtained by different methods are somewhat similar although they vary with experimental parameters. Plasma desorption utilizes high energy (MeV range) fission fragments of 252Cf nuclides. FAB experiments are usually carried out by using high energy beams of Xe atoms. SIMS or LSIMS methods usually utilize 10-35 keV Cs+ ions for surface bombardment, though theoretically any kind of ion (including polyatomic organic species such as C60) can be used. Massive Cluster Impact (MCI) ionization, an extremely soft version of SIMS, applies high energy, multiply charged glycerol cluster ions as the energetic primary beam. Unlike other SIMS methods, MCI can give abundant multiply charged ions, and spectral characteristics much more similar to that of electrospray than to other desorption ionization methods. One low energy type of ion sputtering experiment, chemical sputtering, has also been described. Chemical sputtering is a very efficient experiment that uses low energy ions to release adsorbed molecules at a surface through an electron transfer or chemical reaction event. Laser desorption methods traditionally employ UV lasers (e.g. N2 laser), however utilization of IR lasers, especially the —OH resonant Er:YAG laser (λ=2.94 μm) has become widespread recently.
In order to enhance the ionization efficiency of known desorption and ionization techniques or just simply to make the ionization of certain species feasible, the sample can be deposited onto the surface in a suitable matrix. FAB and LSIMS require the sample to be dissolved in a viscous, highly polar, non-volatile liquid such as nitrobenzyl-alcohol or glycerol. For MALDI applications the sample is cocrystallized with the matrix compound. (Theoretically the individual analyte molecules are built into the crystal lattice of the matrix compound.) MALDI matrices strongly absorb at the wavelength of the laser used, and easily undergo photochemical decomposition which usually involves production of small molecules in the gaseous state.
It was discovered recently, that certain surfaces, e.g. active carbon or electrochemically etched silicon can be used directly as laser desorption ionization (LDI) substrates because these surfaces themselves (or adsorbates on them) strongly enhance the LDI of molecules attached to them. These LDI spectra are similar to MALDI spectra, except for the absence of strong matrix peaks in the former case and the limitation to compounds of somewhat lower molecular weight than traditional MALDI.
Electrospray mass spectrometry was developed as an alternative method to DI for the analysis of non-volatile, highly polar compounds, including macromolecules of biological origin, present in solution phase. Electrospray ionization (ESI) either transfers already existing ions from solution to the gas phase, or the ionization takes place while the bulk solution is being finely dispersed into highly charged droplets. The final gaseous ion formation occurs from these multiply charged droplets by either direct ion evaporation (in the case of low molecular weight ions) or by complete evaporation of solvent from the droplets (in the case of macromolecular ions). One of the main advantages of ESI compared to other DI methods is that ESI can be easily coupled with separation methods such as liquid chromatography or capillary electrophoresis. Another advantage is that it is considerably softer than any of the other DI methods. ESI avoids the need to dry samples or to co-crystalize sample material with a matrix. A further advantageous feature of ESI is the production of multiply charged species out of macromolecular samples. This phenomenon makes macromolecular mass spectrometry feasible using practically any kind of mass analyzer including the quadrupole mass filter, the quadrupole ion trap, ICR, and magnetic sector instruments. This phenomenon of multiple charging has disadvantages too, especially in the analysis of mixtures, since the signal for one analyte is distributed into multiple charge states, which can complicate spectral interpretation. The most serious drawback of ESI compared to MALDI is the limited success of automation of the method. While average MALDI analysis time for a sample can be less than a second, in the case of ESI the shortest achievable time per analysis for a single source system is 20-40 seconds, due to carry over problems.
Although there have been recent advances in ionizing materials for mass analysis, certain unmet needs stand in the way of more widespread commercial use of such techniques. For example, a need exists for a lower-energy desorption ionization method useful in an environment other than a vacuum of the type required by SIMS. Such a desorption ionization method will fill an existing need if it functions at atmospheric pressure and in ambient (uncontrolled) conditions as well as in more controlled environments, such as those found in a laboratory or in a manufacturing facility. There is also a need for such a method that is substantially non-destructive of the sample, provides accurate results rapidly, is capable of ionizing and desorbing samples from a wide variety of surfaces and that avoids the need for pre-treating samples with, for example, a matrix material. Further, there is a need for desorption ionization-based assays sufficiently gentle to be useful on animal tissue, plant tissue and biological materials, for example in connection with in vivo testing for drug metabolites and in testing produce for pesticide residue. There is also a need for forensic assays useful in the rapid, accurate and substantially non-destructive determination of trace materials on both uncontrolled and laboratory surfaces at atmospheric pressure. A need exists for accurate, fast and minimally destructive quality control assays in manufacturing processes, including manufacturing processes in the pharmaceutical industry. There is also a need for fast, accurate clinical assays for components of body fluids such as blood, urine, plasma and saliva and for an improved assay for samples that have been subjected to preparatory separation techniques, such as gel chromatography or binding by ligans. A need also exists for fast assays of microorganisms and bacteria.